Parsing circuit: cannot set a value

[Makubu]

I am a new user of gnark, trying to make a simple example where i generate a proof of knowledge of a discret log. But i encouter the following error: parse circuit: can't set val sk

I guess it is a small error i cannot find

Here is the full code:

package main

import (
	"fmt"
	"math/big"

	"github.com/consensys/gnark-crypto/ecc"
	bls12377 "github.com/consensys/gnark-crypto/ecc/bls12-377"
	bls12377_fp "github.com/consensys/gnark-crypto/ecc/bls12-377/fp"
	sw_bls12377 "github.com/consensys/gnark/std/algebra/native/sw_bls12377"

	"github.com/consensys/gnark/backend/groth16"
	"github.com/consensys/gnark/frontend"
	cs_r1cs "github.com/consensys/gnark/frontend/cs/r1cs"
	"github.com/consensys/gnark/constraint"
)

type discretLog struct {
	sk frontend.Variable
	pk sw_bls12377.G1Affine `gnark:",public"`
}

func (circuit *discretLog) Define(api frontend.API) error {

	// compute the scalar mutliplication
	G1Affine := sw_bls12377.G1Affine{}
	xG := G1Affine.ScalarMulBase(api,circuit.sk)

	api.AssertIsEqual(circuit.pk.X, xG.X)
	api.AssertIsEqual(circuit.pk.Y, xG.Y)

	return nil

}

func main() {

	var sk bls12377_fp.Element
	sk.SetRandom()
	
	G1Affine := bls12377.G1Affine{}
	var skBig big.Int
	sk.BigInt(&skBig)
	pub_key := G1Affine.ScalarMultiplicationBase(&skBig)

	var circuit discretLog
	var err error
	var r1cs constraint.ConstraintSystem
	r1cs, err = frontend.Compile(ecc.BW6_761.ScalarField(), cs_r1cs.NewBuilder, &circuit)
	if err != nil {
		fmt.Println("Err compiling circuit: ", err.Error())
		panic(err)
	}
	fmt.Println("Circuit compiled: ", r1cs)

	pk, vk, err := groth16.Setup(r1cs)

	var assignment discretLog
	assignment.sk = skBig.Int64()
	assignment.pk = sw_bls12377.NewG1Affine(*pub_key)

	fmt.Println("Computing witness")
	witness, _ := frontend.NewWitness(&assignment, ecc.BW6_761.ScalarField())

	publicWitness, _ := witness.Public()
	// generate the proof
	proof, _ := groth16.Prove(r1cs, pk, witness)

	err = groth16.Verify(proof, vk, publicWitness)

}

I switched from BN254 to BW6_761 because I will need some recursion, as explained in the playground :
In-circuit pairing computation needs a SNARK friendly 2-chains of elliptic curves. That is: the base field of one curve ("inner curve)" is equal to the scalar field of the other ("outter curve"). This example use the pair of curves BW6_761 / BLS12_377 More details on the curves here https://eprint.iacr.org/2021/1359 Overrides the default playground curve (BN254) with the curve BW6_761

[Makubu]

The question was not relevant, the only problem was that the circuit field names were not starting with capital letters, hence are not exported.

[ivokub]

Thanks for reporting the solution!